Galileo Galilei’s discovery of the moons of Jupiter. This is a manuscript page, in Italian, on which Galileo first noted an observation of the moons; a full description of them appeared in Sidereus Nuncius in March 1610. For a translation from Sidereus Nuncius click on the picture.

In 1610 Galileo discovered Jupiter's four largest satellitess (moons): Io, Europa, Ganymede, and Callisto. He determined that these moons were orbiting the planet since they would occasionally disappear; something he attributed to their movement behind Jupiter. He made additional observations of them in 1620. (Later astronomers overruled Galileo's naming of these objects, changing his Medicean stars to Galilean satellites.) The demonstration that a planet had smaller planets orbiting it was problematic for the orderly, comprehensive picture of the geocentric model of the universe, in which everything circled around the Earth.

Galileo noted that Venus exhibited a full set of phases like the Moon. Because the apparent brightness of Venus is nearly constant, Galileo reasoned that Venus could not be circling the Earth at a constant distance. By contrast, the heliocentric model of the solar system developed by Copernicus would neatly account for the steady brightness by reason of the much greater distance from the Earth at the time of "full Venus", when the two planets were on opposite sides of the sun such that Venus' illuminated hemisphere faced the Earth.

Galileo made the first European observations of sunspots, although there is evidence that Chinese astronomers had done so before him. The very existence of sunspots showed another difficulty with the perfection of the heavens as assumed in the older philosophy. And the annual variations in their motions, first noticed by Francesco Sizzi, presented great difficulties for either the geocentric system or that of Tycho Brahe.

He was the first to report lunar mountains, whose existence he deduced from the patterns of light and shadow on the Moon's surface. He even estimated their heights from these observations. This led him to the conclusion that the Moon was "rough and uneven, and just like the surface of the Earth itself", and not a perfect sphere as Aristotle had claimed.

alileo observed Neptune in 1611, but believed it to be a star.